The story, which she wrote with longtime collaborator Jon Raymond, is interested in the thin line between activism and terrorism. What happens when a seemingly righteous operation goes wrong and anxiety threatens to overtake ideals? It is the question “Night Moves” asks and answers in chilling ways.

The profiles of the eco-activists at the movie’s center and their efforts could be ripped from the headlines. Josh (Eisenberg) is an organic farmer, part of a cooperative seeking to minimize its footprint on Mother Earth. Dena (Fanning) is a disenchanted, privileged young woman in need of a cause. Her cynicism is smartly laced with philosophical references that make it clear her rebellion has all the right intellectual underpinnings. Ex-Marine Harmon (Sarsgaard) is a more elusive figure, a man in need of a mission and enough of a renegade that environmentalism will do.

The issue that brings them together is water. Images of it flow through the film, from its most natural state to its many uses and abuses by mankind. A dam is the trio’s target. Bringing it down will make a statement. The general populace will be shaken into action. Or so the thinking goes.

Reichardt mines the authenticity to create a tension that is more subtle than sensational, but no less disturbing or stomach-churning for it. As much as the environment looms large over “Night Moves,” this is also an intimate movie about the deeply personal journey taken by its characters. Dena is the linchpin that is loosened just enough to create many of the moral quandaries that will drive the film. It is the most adult role Fanning has had, and the actress sinks her teeth into it.

As the filmmaker is wont to do, Reichardt lets a sense of place settle in before she gets into the heart of the matter. Among the first images we see is the most critical body of water. It lies just underneath the dam. A couple — lovers, perhaps, perhaps not — are on the walkway above.

The scene should be peaceful. There is no talking, just the ambient noises of nature the filmmaker uses so well, though in a far more incidental way than she did in the near-silent meditation on the frontier in “Meek’s Cutoff.” Cinematographer Chris Blauvelt gives the movie a lush, earthy texture without overreaching for perfection. Production designer Elliott Hostetter and costume designer Vicki Farrell extend that understated, organic feel.

But it is an unsettling moment, nevertheless. It’s moody, like Josh, with Eisenberg doing a splendid job of keeping the character, and us, on edge from start to finish. The actor, so good in “The Social Network” as Facebook founder Mark Zuckerberg that it earned him an Oscar nomination, seems to be getting only better at playing mind games.

Reichardt is good at mind games, too. Her portrait of backcountry activism is constructed out of impressions and emotions seeded throughout the film: staying just long enough in the farm commune kitchen to give us a feel for the less politicized eco-concerned; the ones whose actions are guided by the do-no-harm principle. It makes Josh’s agitation more interesting — he’s as much at odds with them as the builders of dams, the exploiters of natural resources.

All the risks of an act of terrorism are weighed along the way, too: the cash-and-barter trade for the necessary goods. The flaws in the reconnaissance work. The waiting for the time to strike. The second thoughts. The rising anxiety chipping away at certainty.

When it all goes wrong and the three scatter to avoid detection, the film hits its stride. How guilt works and whether it is powerful enough to drive one of them to confess is picked apart as “Night Moves” rides the ripple effect of that fear.